Apparatus for causing an aircraft wheel to rotate

ABSTRACT

An apparatus for causing an aircraft wheel to rotate in an airflow includes wind-catching members disposed around a central axis. The wind-catching members are moved between a non-operative, retracted position and an operative-extended position in which the wind-catching members project outside the outer radial periphery of the aircraft wheel. Alternatively, at least one nozzle or vent is provided, centered on the axle of the wheel and arranged to expel air at least partially circumferentially, along with an air supply for providing air to the at least one nozzle or vent. The air supply includes a compressed air source and an air supply passage connecting the compressed air source to the at least one nozzle or vent. Each nozzle or vent is rotatable with the wheel so that the expulsion of air causes rotation of the wheel.

FIELD OF THE INVENTION

The invention relates to apparatus for causing an aircraft wheel torotate.

DESCRIPTION OF RELATED ART

It is well known that aircraft tires need to be replaced at frequentintervals as a result of wear sustained on landing. It is alsorecognized that this wear can be reduced by causing the wheels of anaircraft to rotate, immediately prior to landing, at or near the landingspeed of the aircraft. Many attempts to achieve such wheel rotation havebeen proposed but one of the major difficulties has been devising awheel or attachment which produces sufficient turning moment duringlanding but which does not create problematic drag during take-off.

One way to increase the turning moment during landing without increasingthe drag during take-off is to provide rotating means which are movablebetween a deployed position and a retracted position. In the deployedposition, used during landing, the rotating means are positioned so asto maximize the turning moment applied to the wheel. Increased drag isnot problematic during landing; indeed, it can be beneficial. In theretracted position, used during take-off, drag is minimized along withthe turning moment which is not required on take-off.

Examples of prior art arrangements which utilize this type ofretractable system are illustrated in U.S. Pat. Nos. 2,666,604 and4,732,350. However, because of the need to retract the scoops and vanes,it will be noted that the total area of the scoops and vanes exposed tothe airflow is relatively small. It will also be noted that the vanesand scoops of the prior art are positioned relatively close to theaircraft wheel in their extended positions.

Further examples of prior art, namely U.S. Pat. No. 4,385,739 and U.S.Pat. No. 2,414,849, illustrate methods of causing an aircraft wheel torotate by directing a flow of air, taken either from a compressed airsource or from an engine, onto vanes located on the wheel. However, thepresence of the nozzle which directs the flow of air onto the vanes iscumbersome and increases the drag on take-off.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an aircraft wheel orattachment therefor in which it is possible to expose a relatively largearea of vanes, scoops or other wind-catching members to the airflow.

It is another object of the invention to provide an aircraft wheel orattachment therefor which has rotation means which are extendible awayfrom the axle of the wheel to a relatively great extent to maximize theturning moment applied to the wheel.

It is still another object of the invention to provide an arrangement ofnozzles or the like which does not itself create significant drag upontake-off.

To achieve the above and other objects, the present invention isdirected to an apparatus for causing an aircraft wheel to rotate in anairflow which includes wind-catching members disposed around a centralaxis. The wind-catching members are moved between a non-operative,retracted position and an operative-extended position in which thewind-catching members project outside the outer radial periphery of theaircraft wheel.

The present invention is further directed to an apparatus in which atleast one nozzle or vent is provided, centered on the axle of the wheeland arranged to expel air at least partially circumferentially, alongwith an air supply for providing air to the at least one nozzle or vent.The air supply includes a compressed air source and an air supplypassage connecting the compressed air source to the at least one nozzleor vent. Each nozzle or vent is rotatable with the wheel so that theexpulsion of air causes rotation of the wheel.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings wherein:

FIG. 1 is a plan view of a wheel incorporating apparatus (excludingsails) embodying a first aspect of the invention shown in afully-retracted position;

FIG. 2 is a cross-sectional view taken along the line II--II of FIG. 1;

FIG. 3 is a view similar to FIG. 2 but showing the apparatus in asemi-extended position;

FIG. 4 is a view similar to FIGS. 2 and 3 but showing the apparatus in afully-extended position;

FIGS. 5 and 6 illustrate alternative means for accommodating the wheelof FIGS. 1 to 4 in an undercarriage bay;

FIGS. 7 and 8 illustrate the apparatus of FIGS. 1 to 4 with alternativedesigns of sail attached;

FIG. 9 illustrates an embodiment of a second aspect of the invention ina fully retracted position;

FIG. 10 is a cross-sectional view taken along the line X--X of FIG. 9;

FIG. 11 is a view similar to FIG. 10 but showing the apparatus in asemi-extended position;

FIG. 12 is a view similar to FIG. 9 but showing the apparatus in afully-extended position;

FIG. 13 is a view similar to FIGS. 10 and 11 but showing the apparatusin a fully-extended position;

FIG. 14 is a view similar to FIGS. 10, 11 and 13 but showing theapparatus in a semi-retracted position;

FIG. 15 is a plan view of a wheel incorporating apparatus according to afirst embodiment of a third aspect of the invention;

FIG. 16 is a cross-sectional view taken along line XIV--XIV of FIG. 15;

FIG. 17 is a plan view of a part of apparatus according to a secondembodiment of the third aspect of the invention; and

FIGS. 18 and 19 illustrate further embodiments of the third aspect ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 8 illustrate apparatus 10 according to a first aspect of theinvention in combination with a wheel 12 and pneumatic tire 14. Thewheel 12 consists essentially of a wheel hub 16 and a wheel rim 18 onwhich the pneumatic tire 14 is mounted. The hub 16 is mounted on an axle20 by means of bearings 22 in the normal manner.

Mounted inside the axle 20 is a piston and cylinder arrangement 24. Thepiston and cylinder arrangement 24 is powered by means of an existingcompressed air source or other pneumatic or hydraulic system alreadylocated in the aircraft. Convenient systems which could be utilized arethose used to operate the engine compressor, the de-icing system or thehydraulic braking system. Since the system will only be required tooperate the apparatus of the present invention for a very short timeduring the landing approach, there should be no interference with thenormal operation of any appropriate system. An appropriate connection 26is provided to enable connection with an appropriate hydraulic orpneumatic source.

The piston and cylinder arrangement 24 is utilized to cause telescopicextension and retraction of the remainder of the apparatus 10 in thelongitudinal direction of the axle 20. A telescopically extending rod 28is designed to support the remainder of the apparatus 10. The remainderof the apparatus 10 essentially comprises a central support member 30extending substantially diametrically across the wheel hub 16. At eitherend of the support member 30 are pivotable connections 32, each of whichconsists essentially of a U-shaped housing 34 supporting an elongatepivot 36 between its arms. Mounted on the elongate pivot 36 between thearms of each U-shaped housing 34 is a toothed wheel 38. The toothedwheel 38 is arranged to engage either with a vertically extending spurrack 40 which extends from the wheel rim 18 in the direction of the axle20, or with an additional rack 42 located on an inclined surface of thewheel rim 18.

Fixedly attached to a first end of the elongate pivot 36, is an outerarm 44. Hingedly attached to the distal end of the outer arm 44 remotefrom the elongate pivot 36 is a bracing arm 46. Rotatably attached tothe other end of the elongate pivot 36 is an upper arm 48. Theconnection between the upper arm 48 and the elongate pivot 36 is by wayof gearing or a screw thread such that rotation of the elongate pivot 36through a predetermined angle results in rotation of the upper arm 48through substantially half of that predetermined angle. Alternatively, aslip and lock connection can be provided between the elongate pivot 36and the upper arm 48.

The mounting of the pivotable connections 32 on the support member 30are such that a small amount of displacement therebetween is allowable.Effectively, both U-shaped housings 34 can be displaced radially withrespect to the wheel 12 towards the axle 20 so as to allow the toothedwheels 38 to become disengaged from the spur racks 40. The purpose ofthis feature will be described later. Means (not shown) for preventingthe unintentional pivoting of the U-shaped housing 34 may be providedand are releasable by means of release buttons 50 operated by contactwith the lower surface of the support member 30. Also, latches 52 may beprovided on the support member 30 or on any other convenient part of thewheel 12 for holding the various arms 44, 46, 48 in their retractedpositions.

The operation of the apparatus 10 will now be described with referenceto FIGS. 2, 3 and 4. The sail members normally attached to the arms havebeen omitted from these Figures for the purposes of clarity.

FIG. 2 illustrates the apparatus 10 in its fully retracted position. Thetelescopic rod 28 is retracted within the axle 20 under the action ofthe piston and cylinder arrangement 24. The support member 30 isretracted as far as possible towards the wheel 12, thereby maintainingcontact with the release buttons 50. This allows the U-shaped housing 34to be rotated with respect to the support member 30 so that the toothedwheels 38 are in interengaging contact with the additional racks 42 andout of contact with the spur racks 40. The toothed wheels 38 aretherefore rotated as far as possible in a direction which maintains thevarious arms 44, 46, 48 firmly pressed towards the support members 30.The latches 52 maintain the various arms 44, 46, 48 in this retractedposition.

When the apparatus 10 is to be deployed, compressed air or otherhydraulic or pneumatic fluid is introduced into the piston and cylinderarrangement 24 via the connection 26. The operation of the piston andcylinder arrangement 24 causes the telescopic rod 28 to extend in thelongitudinal direction of the axle 20, thereby moving the entireapparatus 10 in the same direction. The movement of the support member30 out of contact with the release buttons 50 locks the pivotableconnections 32 into the positions illustrated in FIG. 3. This lockingcoincides with the movement of the toothed wheels 38 outwardly along theadditional racks 42 and into engagement with the spur racks 40. Themovement of the toothed wheels 38 along the racks 40, 42 causes rotationof the toothed wheels 38 thereby rotating the elongate pivots 36. Thisrotation causes corresponding rotation of, initially, the outer arms 44.The upper arms 48 either do not rotate at all initially or rotate at aslower rate, depending upon the type of connection between the upperarms 48 and the elongate pivots 36. The bracing arms 46, being pivotallyconnected to the distal ends of the outer arms 44, are carried therewithduring the rotation of the outer arms 44. A semi-extended position isillustrated in FIG. 3.

FIG. 4 illustrates the fully extended position which is reached when thetelescopic rod 28 reaches its maximum extension. In this position, thetoothed wheels 38 have reached the upper extremities of the spur racks40 and the outer arms 44 extend in a generally radial direction withrespect to the wheel 12. The bracing arms 46 are configured andconnected to the outer arms 44 in such a way that they extend generallycircumferentially with respect to the wheel 12. Also, the upper arms 48extend generally in the longitudinal direction of the axle 20.

In this fully extended position, the apparatus 10 is used to generaterotation of the wheel 12 and tire 14. This is achieved by the fact thatsail members 54 are attached to the various arms 44, 46, 48. Alternativearrangements of sail members 54 are illustrated in FIGS. 5 and 6.Rotation of the apparatus 10 by virtue of the action of the air flowduring flight causes rotation of the wheel 12 and tire 14. This rotationsubstantially reduces wear and tear on the tire 14 during landing.

The sail members 54 illustrated in FIG. 5 are flexible in nature inorder to allow them to be folded when the apparatus is retracted. In thefully extended position, the sail members 54 essentially form a cup orscoop shape in order to catch the air flow and cause rotation in therequired direction. The sail members 54 are fixedly connected to theouter arms 44, the bracing arms 46 and the upper arms 48. Furthermore,an inner sail portion 56 is anchored to the apparatus, preferably to thesupport member 30.

Each sail member 54 can include additional bracing members 58 in orderto reduce any risk of the sail member 54 failing to unfold properly orassume the desired cup or scoop shape. Additionally, weighting means 60can be incorporated into the sail member 54 at or or near the pointfurthermost from the outer arm 44. The weighting means can be formedintegrally with the sail members 54 in the form of additional weights orother weighting means and is operated purely by the action ofcentrifugal forces as the apparatus 10 rotates.

FIG. 6 shows an alternative sail arrangement which incorporates asmaller sail area. In this arrangement, the sail members 54 are arrangedat the radially outermost ends of the arrangement of arms. Connectingwires or cords 62 can be used to ensure adequate bracing and thepreviously described weighting means 60 are also included. The advantageof the arrangement illustrated in FIG. 6 is that the smaller sailmembers produce less drag during operation whilst still ensuring thatthe wheels and tires assume the desired rotational speed.

In order to return the apparatus 10 to the fully retracted positionillustrated in FIG. 2, and this must clearly take place before theaircraft actually lands in order to avoid damage to the outer andbracing arms 44, 46 and sail members 54, the process described abovemust be reversed. The piston and cylinder arrangement 24 is againactivated so as to bring the telescopic rod 28 into the retractedposition. The retraction of the rod 28 causes the toothed wheels 38 tomove down the spur racks 40 and the rotation of the toothed wheels 38causes the arms 44, 46, 48 to rotate together with the elongate pivots36 back into their retracted positions. As the toothed wheels 38 reachthe lower ends of the spur racks 40, the support member 30 comes intocontact with the release buttons 50 thereby allowing the U-shapedhousings 34 to pivot with respect to the support member 30. Continuedretraction of the rod 28 causes the toothed wheels 38 to transfer fromthe spur racks 40 onto the additional racks 42, thereby releasing theprevious connection between the toothed wheels 38 and the spur racks 40.At this point, the arms 44, 46, 48 are all in their fully retractedpositions and the sail members 54 are securely folded within thearrangement of arms 44, 46, 48. This retraction can easily take placeimmediately before landing without substantially reducing the acquiredrotation of the wheel 12.

One further point requires to be addressed in connection with thisembodiment and this is the accommodation of the spur racks 40 during theperiod when the undercarriage is raised for normal flight. Normally, thecompartments within which the undercarriage is stowed during flight isprecisely shaped to the requirements of the undercarriage and noadditional space is allowed. If the present invention is to be utilizedon aircraft already in existence, then provision must be made for theaccommodation of the spur racks 40 which project beyond the normal outerplane of the wheel arrangement. Two alternative proposals are set out inFIGS. 7 and 8.

If the spur rack 40 is to be rigidly fixed to the wheel rim 18 with nomovement possible, then the undercarriage bay 64 must contain anappropriate recess 66 (FIG. 7). However, it must be borne in mind that,because the rotational position of the wheel 12 is infinitely variableafter take-off, the recess 66 must take the form of an annular grooveextending around the undercarriage bay. Of course, if the angularposition of the wheel could be fixed after take-off, then a single blindbore for each spur rack 40 would be acceptable. However, additionalapparatus to fixedly select the angular positioning of the wheel aftertake-off would then be required.

A further alternative version is illustrated in FIG. 8. In thisarrangement, no recess is provided in the undercarriage bay 64 becausethe spur rack 40 is retractably mounted in the wheel rim 18. Biassingmeans 68, normally in the form of a compression spring, bias the spurrack 40 into the operational position illustrated in FIGS. 2, 3 and 4.Locking means (not shown) retain the spur rack 40 in this operationalposition. However, means for releasing these locking means can beprovided, conveniently at the distal end of the spur rack 40. Thus, asthe undercarriage is retracted into the undercarriage bay 64, the distalend of the spur rack 40 contacts a predetermined area of theundercarriage bay thus releasing the locking means. The spur rack 40 isthen retracted into the wheel rim 18 against the action of the biassingmeans 68. As soon as the undercarriage is lowered, the spur rack 40assumes its operational position as before.

Either of these arrangements would be suitable to accommodate aprojecting spur rack in the undercarriage bay.

It will be appreciated that the apparatus described above will causevery little additional drag during take-off in view of the fact that itwill be fully retracted. The apparatus will only come into effect as theaircraft makes its approach for landing. The apparatus would be extendedduring the landing approach and retracted immediately before finaltouch-down. This will give the aircraft wheels the maximum opportunityto attain a rotational speed appropriate to the landing speed of theaircraft. Regulating means can be provided if necessary to avoid thewheels attaining a rotational speed which is too high for the aircraftconcerned. Alternatively, the area of the sail members can be varied inorder to match the attainable wheel rotation speed to the aircraft inwhich the apparatus is deployed.

The apparatus can be formed from various high strength, low densitymaterials such as those commonly in use in aircraft manufacture today.Furthermore, the apparatus can easily be fitted to existing aircraftwith minimum disruption.

An embodiment of a second aspect of the invention is illustrated inFIGS. 9 to 14. In general, the Figures illustrate the apparatus 100 incombination with a wheel 102, consisting of a hub 104 and a rim 106 onwhich a pneumatic tire 108 is mounted, the wheel hub 104 being mountedin a conventional manner on an axle 110 by way of bearings 112.

The axle 110 incorporates a hollow chamber 114 in which a hollow supportmember 115 is telescopically movable. A hollow rod member 116 formingpart of the axle 110 extends coaxially with the chamber 114 and supportmember 115 towards the apparatus 100. A valve for emitting compressedair or other hydraulic or pneumatic fluid is arranged in the distal endof the rod member 116. The valve is connected in any convenient mannerto an existing pneumatic or hydraulic system already incorporated withinthe aircraft.

An upper housing 118 is rotatably mounted on the support member 115above the wheel rim 106. The upper housing 118 is connected to the wheel102 in that rotation of one about the axle 110 will cause rotation ofthe other. An upper reel 120 is also rotatably mounted with respect tothe support member 115 and a braking member 122 is located on the sideof the upper reel 120 remote from the upper housing 118. The brakingmember 122 is axially moveable with respect to the support member 115but not rotatable with respect thereto.

The upper housing 118 contains an interior chamber 124 which is incommunication with a compressed air source (not shown) via the valve inthe rod member 116. At the outer circumference of the upper housing 118,a plurality of air vents 126 (see FIG. 12) are provided and a rotatablymounted fan arm 128 is associated with each air vent 126. The rotarymounting of each fan arm 128 is provided by means of a verticallyextending axle 130 located adjacent the respective air vent 126 andabout which the respective fan arm 128 is freely rotatable.

Located between the fan arms 128 and the wheel hub 104 is a freelyrotatable lower reel 132. Also, tension cables 134 extend from theinterior wall of the reel rim 106, to the upper reel 120, passingthrough the lower reel 132 and the distal ends of a plurality of the fanarms 128. The purpose of this arrangement will be described below.

The fan arms 128 are generally angled or curved as shown in FIG. 9 sothat, in the retracted position, they can overlap with one another andgenerally follow the shape of the upper housing 118. This minimizes anydrag effect during take-off. Each fan arm 128 consists of a plurality ofsubstantially planar fan members 136 lying generally tangentially to theupper housing 118. These fan members 136 are illustrated most clearly inFIG. 12. The fan arms 128 also incorporate sail members 140 which extendgenerally perpendicular to the longitudinal axis of the apparatus 100 inthe fully extended position.

FIGS. 9 and 10 illustrate the apparatus 100 in its fully retractedposition. The support member 115 is fully retracted and the fan arms 128are nested closely about the upper housing 118. The support member 115is biassed into this retracted position by means of, for example, acompression spring 138 located inside the axle 110.

When the apparatus is to be extended, compressed air is caused to flowthrough the valve in the distal end of the rod member 116. The pressurebuild-up in the chamber 124 causes the apparatus 100 to movelongitudinally with respect to the axle 110 by telescopic sliding of thesupport member 115 in the hollow chamber 114. The apparatus 100therefore extends together with the support member 115 away from thewheel hub 104. Simultaneously, the compressed air introduced into theinterior chamber 124 of the upper housing 118 via the telescopic rod 116is used to maintain the braking member 122 out of contact with the upperreel 120 as the apparatus moves into an extended position. Asemi-extended position is illustrated in FIG. 11.

The compressed air entering the interior chamber 124 is allowed to exitvia the air vents 126 which are arranged around the circumference of theinterior chamber 124. The air vents 126 direct the compressed air ontothe fan members 136 of the fan arms 128. This causes the fan arms 128 torotate about the vertical axles 130 and move into the fully extendedposition illustrated in FIGS. 12 and 13. In this fully extendedposition, continued expulsion of compressed air through the air vents126 causes the compressed air to impinge upon the fan members 136 andthis in turn causes the fan arms 128 to rotate thus causing rotation ofthe upper housing 118 and thereby causing rotation of the wheel 102. Thesail members 140 encourage the compressed airstream to impinge upon thefan members 136 without becoming diverted.

When it is desired to retract the apparatus 100 immediately prior totouch-down, the following steps are taken. Initially, the compressed airsupply is cut off. A throttle valve (not shown) in the hollow chamber114 ensures that the apparatus 100 retracts relatively slowly to allowtime for the fan arms 128 to return to their retracted positions beforethe upper housing 118 returns to its own retracted position. However,the cutting off of the compressed air supply means that the brakingmember 122 is no longer maintained spaced apart from the upper reel 120.The braking member 122 therefore comes into frictional contact with therotating upper reel 120 and slows the rotation thereof. The tensioncables 134, which are fixedly connected to the circumference of theupper reel 120, are therefore effectively wound onto the upper reel.This places the tension cables 134 under increased tension and forcesthe fan arms 128, to whose distal ends the tension cables 134 areattached, to return to their retracted positions. The purpose of thelower reel 132 is to take up any slack in the tension cables 134 duringthe extension and retraction of the fan arms 128. Ideally, the lowerreel 132 is kept under the action of biassing means so as to allowequilibrium to be maintained as far as possible without any slack beingavailable.

As the fan arms 128 are moving between their extended and retractedpositions, the apparatus 100 is also retracting in the axial directionof the wheel axle 110. The action of the throttle valve can be selectedto ensure that the fan arms 128 do not come into contact with either thetire 108 or the wheel rim 106.

If desired, the wheel rim 106 or, indeed, the pneumatic tire 108 canincorporate relatively small wind catching members 142 which aredesigned to encourage the wheel 102 to rotate in a direction opposite tothat required for landing. This feature is utilized during the extensionof the apparatus 100 in order to allow the tension in the tension cables134 to be relaxed as the fan arms 128 are moving from their retractedposition to their extended position. Thereafter, the effect of the fanarms 128 will override any effect of the wind catching members 142.

Further embodiments similar to that described with reference to FIGS. 9to 14 are shown in the remaining Figures. However, in the remainingembodiments, no extension of the apparatus is required. Theseembodiments are therefore relatively simple in that they do notincorporate any moving parts.

Essentially, in each embodiment, a compressed air chamber or passageway150 is provided. Looking firstly at FIGS. 15 and 16, the chamber isbounded on one side by an upper cover 152 and by a lower passageway 154on the other. At the radial extremes of the compressed air chamber 150,vents 156 are provided through which the compressed air introduced intothe chamber 150 can escape. These vents 156 are arranged such that atangential element of force is created, thereby ensuring that the wheelis rotated as compressed air passes through the vents 156. Arrangementsdifferent from that shown in FIG. 16 are, of course, possible andequally effective.

The lower passageways 154 illustrated in FIG. 16 are arranged to passthrough the wheel rim into the pneumatic tire cavity before emergingagain on the side of the tire remote from the cover 152. This is done toavoid interference with the braking system. It also reduces the risk ofan imbalance of the wheel occurring.

Turning now to the embodiments shown in FIGS. 17 to 19, it will beappreciated that FIGS. 18 and 19 each illustrate two various alternativearrangements. However, these embodiments are all based on the principleof the compressed air chamber 150 as illustrated in FIGS. 15 and 16.FIG. 17 is a plan view of the air chamber illustrated on the left handside of FIG. 18. In this embodiment, compressed air is introduced to thechamber 150 via the axle. The compressed air passes along the radialpassages 158 and then along the generally tangential passages 160 beforeexiting to atmosphere. The tangential arrangement of the passages 160causes the wheel to which the chamber 150 is attached to rotate ascompressed air is expelled.

It is particularly advantageous if the chamber 150 described above isused in conjunction with a wheel having two adjacent tires attached to asingle wheel rim. The chamber 150 can be arranged so that the generallytangential passages 160 open in the area immediately between the twotires. This reduces the risk of the passages 160 becoming inadvertantlyblocked by foreign matter, although care must be taken to ensure thatthe passages 160 do not extend radially to such an extent that they maybecome damaged during landing.

A similar but alternative arrangement is illustrated on the right handside of FIG. 18. The only difference between this arrangement and thatdescribed above is the fact that the passages 162 extend substantiallyradially with respect to the wheel rim and incorporate tangential vents164 at their distal ends. Again, the vents are effectively shielded fromdamage by the dual tires and are positioned sufficiently near to theaxle to avoid damage on landing.

FIG. 19 illustrates two further variations on the same theme. In theleft hand side of FIG. 19, the radially outward area of the chamber 150is essentially incorporated into the tire. The tire 170 is mounted on asingle wheel rim but incorporates two separate tubes 172,174. The wallsof the tubes 172,174 also form the walls of the radial passage 176 whichis located between the tubes 172,174. A tangential vent 178 is arrangedin the end of the passage 176. When compressed air introduced into thechamber 150 through the axle is allowed to escape through the vents 178,the wheel as a whole is caused to rotate.

As a final example, the outer wall of the tire can incorporate passageswhich will create the desired effect. In the right hand side of FIG. 19a tire 180 is mounted on a wheel rim. The tire 180 has a passage 182moulded into the outer wall thereof which is in communication with thechamber 150 via a connection in the wheel rim. In the radially outerwall of the tire 180, ie remote from the wheel rim, vents 184 arearranged which allow compressed air travelling down the passage 182 toescape to atmosphere. The vents 184 are arranged so as to produce atangential element of flow in order to ensure that the passage of airtherethrough creates a rotational movement of the wheel. The passage 182is kept completely separate from the tire proper; ie. there is asubstantial dividing wall between the tire cavity 186 and the passage182. The vents 184 can also be protected by means of shields (not shown)to prevent damage thereto on landing.

It will be appreciated that all of the arrangements illustrated in FIGS.18 and 19 are automatically balanced provided that theradially-extending passages are equiangularly spaced. It will also beappreciated that none of the embodiments illustrated in FIGS. 15 to 19contain moving parts and therefore the cost of maintaining and repairingsuch parts is eliminated.

I claim:
 1. Apparatus for causing an aircraft wheel to rotate in anairflow, comprising a plurality of wind-catching members disposed abouta central axis, wherein driving means are provided for drivably movingthe wind-catching members between a nonoperative, retracted position andan operative, extended position in which the wind-catching membersproject radially beyond the outer radial periphery of the aircraftwheel.
 2. Apparatus as claimed in claim 1, wherein, in the extendedposition, the wind-catching members project at least as far from thecentral axis as the outer radial periphery of a tire attached to theaircraft wheel.
 3. Apparatus as claimed in claim 1, wherein, in theretracted position, the wind-catching members do not project beyond theouter periphery of the aircraft wheel.
 4. Apparatus as claimed in claim1, wherein, during movement between the retracted and extendedpositions, the wind-catching members are displaced both radially andaxially with respect to the central axis.
 5. Apparatus as claimed inclaim 4, wherein the wind-catching members are mounted on a centralmounting portion which is displaced axially during movement between theretracted and extended positions.
 6. Apparatus as claimed in claim 5,wherein, on movement from the retracted position to the extendedposition, the wind-catching members move radially subsequent to theaxial movement of the central mounting portion.
 7. Apparatus as claimedin claim 5, wherein, on movement from the retracted position to theextended position, the wind-catching members move radiallysimultaneously with the axial movement of the central mounting portion.8. Apparatus as claimed in claim 1, wherein the driving means comprisesan hydraulic or pneumatic arrangement housed or housable within a hub oraxle of the aircraft wheel.
 9. Apparatus as claimed in claim 8, whereinthe hydraulic or pneumatic arrangement is connected or connectable to asupply of hydraulic or pneumatic fluid in the aircraft and normally usedfor purposes other than the movement of the wind-catching members. 10.Apparatus as claimed in claim 1, wherein the wind-catching memberscomprise articulated arm members and flexible sail members. 11.Apparatus as claimed in claim 10, wherein the sail members incorporatereinforcement means.
 12. Apparatus as claimed in claim 11, wherein thereinforcement means comprise struts disposed in or on the flexible sailmembers.
 13. Apparatus as claimed in claim 10, wherein the sail memberscomprise weighting means located substantially centrally of the flexiblesail members.
 14. Apparatus as claimed in claim 13, wherein theweighting means are located in pockets in the flexible sail members. 15.Apparatus as claimed in claim 10, wherein, in the retracted position,the arm and sail members are folded so as to minimize drag. 16.Apparatus as claimed in claim 10, wherein, in the extended position, thearm and sail members present a relatively large area to the passingairflow such that rotation of the aircraft wheel is induced. 17.Apparatus as claimed in claim 10, wherein the wind-catching members areequispaced about the central axis, wheel hub or axle.
 18. Apparatus asclaimed in claim 17, wherein two wind-catching members are provided. 19.An aircraft wheel incorporating the apparatus of claim
 1. 20. Anaircraft having at least one wheel as claimed in claim
 19. 21. Apparatusfor causing an aircraft wheel to rotate, comprising at least one nozzleor vent located on the circumference of a circle centered on the axle ofthe aircraft wheel and arranged to expel air at least partiallycircumferentially with respect to the said circle, and air supply meansfor providing air to the at least one nozzle or vent for expulsiontherethrough, the air supply means comprising a compressed air sourceand an air supply passage connecting the compressed air source to the atleast one nozzle or vent, the at least one nozzle or vent beingrotatable with the aircraft wheel such that expulsion of the air throughthe at least one nozzle or vent causes rotation of the aircraft wheel,wherein, when the at least one nozzle or vent is in use, the at leastone nozzle or vent is located radially beyond the outer radial peripheryof the aircraft wheel.
 22. Apparatus as claimed in claim 21, wherein theair supply passage passes through the axle of the wheel.
 23. Apparatusas claimed in claim 22, wherein a portion of the air supply passage isformed by a chamber located within a hub of the wheel.
 24. Apparatus asclaimed in claim 22, wherein a portion of the air supply passage islocated inside a tire attached to a rim of the wheel.
 25. Apparatus asclaimed in claim 24, wherein the portion of the air supply passagelocated inside the tire is formed integrally with the said tire. 26.Apparatus as claimed in claim 24, wherein at least some of the nozzlesor vents are located on the tire.
 27. Apparatus as claimed in claim 21,wherein at least some of the nozzles or vents are located on the wheelor an attachment therefor.
 28. Apparatus as claimed in claim 21, whereina plurality of nozzles or vents are equispaced about the circumference.29. Apparatus as claimed in claim 21, wherein the at least one nozzle orvent is arranged to expel air substantially tangentially with respect tothe said circle.